EP0399111A1

EP0399111A1 - Cathodic protection of subterranean pipes

Info

Publication number: EP0399111A1
Application number: EP89305190A
Authority: EP
Inventors: Stephen Roy Willis
Original assignee: Severn Trent Water Ltd
Current assignee: Severn Trent Water Ltd
Priority date: 1987-11-19
Filing date: 1989-05-23
Publication date: 1990-11-28
Also published as: GB2212514B; GB2212514A; GB8727078D0

Abstract

Cathodic protection is provided to a subterranean pipe (1) by the known method of making a small (non-man-entry) excavation to expose the top surface of the pipe, descaling and/or abrading the exposed pipe surface using tools (T₁, T₂) working from ground level (10), fixing an electrical conductor to the prepared pipe surface and connecting the conductor to a sacrificial anode. The invention is concerned with the provision of a liner sleeve (3) in the excavation, having near its lower end a transverse member (4) with an aperture (6) which locates and guides the tools. The vertical position of the member (4) can be adjusted.

Description

This invention relates to the cathodic protection of subterranean pipes, especially (but not exclusively) cast iron water and gas pipes.
It is known to reduce the rate of corrosion of subterranean iron pipes by connecting them electrically to a sacrificial anode buried in the ground nearby. In the case of new pipes, this cathodic protection can be provided when the pipes are laid, but where existing installations are concerned, excavation is necessary both to expose the pipe surface to connect a wire to it and to bury the sacrificial anode. Excavation large enough for entry of a man,e.g. 2m x 1.5m x depth of pipe is relatively costly not only because of the volume of earth etc. to be moved but also because of the substantial cost of reinstatement when the work is complete. In fact, the cost of providing cathodic protection in this way generally equals or exceeds the cost of completely replacing the pipe, and so is usually not worthwhile. However, if cathodic protection for existing installations could be provided more economically, much use could with advantage be made of it.
In European Patent Specification No. 0 235917 of the Water Research Centre there is described a method of tapping subterranean pipes in which, in some embodiments small excavations, too small for entry of a person, are used to expose the pipe. In one such procedure, pins or studs are brazed to the excavated pipe to locate a saddle on the pipe prior to tapping. These pins or studs can be joined to an electric wire connected to a sacrificial anode to provide cathodic protection. However, whilst substantial economies can be achieved in this way, i.e. by using small excavations e.g. 25 to 30 cm in diameter, very serious problems arise in making a good electrical connection to the surface of the pipe whilst working from ground level down a very narrow hole. For example, it is very difficult properly to clean a small area of the pipe surface to receive the connection, and the making of the connection, whether by brazing or other wise, can also be difficult.
Also U. S. Patent Specification No. 3 869361 discloses a method of making electrical connections to underground pipes down a relatively narrow excavation, using long-handled tools to clean an area of the exposed upper surface of the pipe (for example by the use of a needle scaler) and then to weld onto the cleaned surface a stud for connection to (or it may already be connected to) a sacrificial anode.
In practice such an operation can be very tricky as it is difficult to manipulate, from above ground level, long-handled tools to make the necessary weld at just the right place on the convex exposed surface of the pipe two or more metres below, in conditions of very restricted access and visibility.
The same applies to the cleaning step; it is extremely difficult to descale and abrade the small area of pipe surface working down the hole with long tools from ground level. The tools tend to wander over and off the convex surface of the pipe and are very easily contaminated and/or clogged with soil.
We have now found a way of overcoming or mitigating these problems whereby a good electrical connection can be reliably made, and cathodic protection provided at a cost substantially less than the cost of the pipe replacement.
According to the invention there is proposed a method of providing cathodic protection to an underground pipe which comprises the steps of making a small excavation from ground level down to the upper surface of the pipe to expose that upper surface, cleaning a small area of that surface and securing to it an electrical conductor which is connected, already or subsequently, to a sacrificial anode, the cleaning and securing steps being performed with long-handled tools from ground level, characterised in that
after the excavation has been made, and prior to the cleaning step, a cylindrical liner is inserted in the excavation and there is provided within the liner a transverse member having therein an aperture of limited dimensions through which the subsequent steps are performed, the aperture forming a guiding and locating means by which the tools used in those subsequent operations confine their actions to the required area of the surface of the pipe.
The transverse member can be a disc with the aperture in the centre, but to give greater flexibility, for example if the liner is found to be placed not truly symmetrically over the crown of the pipe, the aperture can be formed in a plate which is itself transversely adjustable in position in relation to the transverse member.
By restricting the lateral movement of the tools the aperture ensures that the tools confine their action to the required limited are of the pipe and also it reduces greatly the likelihood of the tools picking up earth from the sides of the excavation.
The apertured transverse member furthermore positively locates the tools laterally in relation to the pipe without the need for any clamps, jaws or other devices engaging the pipe itself, as in previous proposals.
The liner may have in its wall a series of vertically spaced apertures, in any one of which a cam plate mounted on the transverse member can be caused to engage to locate the member. The cam plate can be controlled by manipulation of a rod which extends upwards to ground level, or at any rate to the neighbourhood of the upper end of the liner.
Preferably there are two such sets of apertures on the wall of the liner diametrically opposite, and correspondingly two cam plates and control rods.
To prevent the transverse member rotating about the axis of the liner the latter has a longitudinally extending rib engaged by a notch in the periphery of the member; preferably there are two such ribs and notches, diametrically opposed.
The liner or sleeve will preferably be a relatively snug fit in the hole, but if not it can for example be temporarily jammed in position. The sleeve not only provides a way of locating the transverse member but it also serves to prevent earth falling from the sides of the hole onto the working area.
The liner is preferably a plastics sleeve(but it can be of other materials). It is normally removed from the excavation after making the electrical connection, and re-used.
The amount and type of surface preparation necessary will depend on the nature of the proposed electrical connection. However, although the connection can be made in a number of ways, e.g. by brazing, welding or by conductive adhesives, in general both descaling and abrasive cleaning of the pipe surface will be desirable. For the descaling purposes, we have found an air-powered needle descaler to be appropriate, fitted of course with an elongate handle and controls as necessary for operation downhole under manual control from ground level. For abrasion, a sand disc can suitably be used in an air powered die grinder. Again the tool requires a long handle and a remote control operation downhole manually from ground level. Both these tools, and other similar tools, can be extremely difficult to use without employing guide means in the form of the transverse member previously described.
After the necessary cleaning of the area of the pipe surface, the electrical connection thereto can be made (step (c)). As stated above, we prefer to use brazing, i.e. to braze a pin to the pipe surface, the pin preferably already sacrificial anode. However, other techniques can be used as mentioned above, although they are not generally as advantageous as brazing.
Where brazing is to be effected, the brazing equipment used is generally standard except that the pins need to be modified for connection to the electric wire, and the pin brazing unit has to be mounted for operation downhole remote from the operator at ground level. Modification of the standard pins can be achieved by, for example, drilling a hole through the pin and soldering and crimping the electric cable thereto. During brazing, the earth connection can be made using, for example, a magnet on a long reach pole. Again, brazing is greatly facilitated by the use of a guide means as previously described.
Finally, the sacrificial anode ('which may be of conventional design) is buried in an appropriate position, with the electrical cable connecting it to the brazed pin (or other connection) on the pipe. The anode may be in a separate nearby small excavation, or it may be placed in the main excavation during infill.
Depending on the spacing of the anode from the pipe, and the situation of the site, e.g. roadway, verge or open country, a number of options are available as to the method of work, and attachment of anode to pipe. Thus:

i) if the holes are in the open country, the anode may be attached (via the cable) to the brazing pin before installation. The pin would be brazed to the pipe and the anode then lowered into position. The cable joining the pipe and anode could be laid in a shallow trench linking the two holes.
ii) if the pipe and anode are to be placed further apart, or the installation is within a roadway, the connecting trench may not be desirable. A small connecting hole could be bored between the two holes, and if the anode were installed first, the brazing pin and cable could then be drawn through this small bore to the pin installation hole, the pin then being brazed as before.
iii) the pin and anode may be installed each with a lead attached but as yet unconnected. The wires can then be joined after installation either using a permanent solder joint, or a plug and socket within a small inspection chamber to allow the anode to be checked periodically.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein:

Fig 1 is a schematic sectional view of an excavation to a pipe, with the sleeve and tool guide in position;
Fig 2 is a schematic view of the cathodically protection pipe arrangement;
Fig 3 is similar to Fig 1 but illustrates the way in which the aperture in the guide means laterally confines the elongate tools;
Fig 4 is a top plan view of one embodiment of sleeve;
Fig 4B is a section of the line X-X of Fig 4A;
Fig 5A is a top plan view of one embodiment of tool guide; and
Fig 5B is a section on the line Y-Y of Fig 5A.

Referring to the drawings, Fig 1 shows a subterranean pipe 1 with a small excavation to reveal its crown area 2. A liner in the form of a cylindrical sleeve 3 is snugly fitted into the excavation, with its upper end 3a slightly above ground level 10. Within sleeve 3 is located a tool guide comprising a plate 4 having an aperture 6, and a pair of elongate control rods 5 extending out of the sleeve 3. Projections 7 are provided to selectively engage in corresponding recesses provided in the sleeve 3, to locate the plate 4 at the desired height above surface 2 for the particular tool (not shown) to be used. After cleaning surface 2 and, for example, brazing a pin on surface 2, the guide and sleeve 3 are withdrawn, and the excavation filled.
Fig 2 shows the final result. Pipe 1 has a pin 20 brazed to its crown 2, with a cable 21 connecting the pin to sacrificial anode 22 spaced from pipe 1.
Referring to Fig 3, the arrangement shown is essentially as in Fig 1 (like numerals indicate like parts), but the extreme lateral positions (not normally adopted) of two tools T₁, T₂, are shown. As can be seen, even at these extremes, the working heads H₁, H₂, are still in the crown area X where the work is required.
Figures 4A and 4B show in some detail a cylindrical sleeve 3. The sleeve is provided internally with two lengthwise ribs 30, 31 extending parallel and being diametrically opposed. These ribs serve as guides to the tool guide of Fig 5. One would be enough but two are preferred. Also, the sleeve 3 has a series of pairs of diametrically opposed slots 32, 33, 34, 35 (only one of each pair is visible in Fig 4B) to receive cam plates of the guide of Figs 5. It is to be noted that each pair of slots is at a different distance from lower end 36 of sleeve 3, so that the tool guide can be mounted at different heights in the sleeve.
The tool guide of Figs 5 comprises a generally circular plate member 40 having two diametrically opposed peripheral recesses 41, 42 in which ribs 30, 31 are to be received as the guide is moved into the sleeve 3. The ribs this locate the plate 40 with respect to the sleeve. Extending from the plate on one face are the two elongate control rods 5, from diametrically opposed positions of plate 40 close to the periphery of the plate and at positions about 90° to the positions of recesses 41, 42. Each rod passes through a bore in the plate and is fixed (e.g. by nut 45) eccentrically to a respective cam member 43, 44 lying below the plate. Rotation of each rod causes the respective cam plate to turn. Each cam plate is shaped so that, in one position (as drawn) it does not project outwardly of the plate periphery but in another position it does project. In this way, the guide can be lowered down sleeve 3 with the cams retracted and then, upon rotating rods 5, the cams can be caused to project and engage a chosen pair of slots 32-35, to fix the guide in the sleeve.
The plate 40 has an aperture 50 therein. Mounted to lie over parts of the aperture is an apertured plate 51. Holes and fixings (52, 53) are provided to locate the plate 51 in any of a series of positions. This enables the plate aperture position to be adjusted. This is important because, in use, the excavation may not expose the pipe crown exactly centrally of the liner sleeve 3. It is then important to be able to guide the working tools into an area (on the pipe crown) which is off-centre of the sleeve 3. This facility if provided by adjusting the position of plate 51 on plate 40 so as to present a through aperture 'Z' in exactly the right position. Thus, the working aperture Z is the through aperture formed by apertured plate 51 and aperture 50 of guide plate 40.

Claims

1. A method of providing cathodic protection to an underground pipe which comprises the steps of making a small excavation (3) from ground level down to the upper surface (2) of the pipe (1) to expose that upper surface, cleaning a small area of that surface and securing to it an electrical conductor (21) which is connected, already or subsequently, to a sacrificial anode (22), the cleaning and securing steps being performed with long-handled tools (T1, T2) from ground level, characterised in that
after the excavation has been made, and prior to the cleaning step, a cylindrical liner (3) is inserted in the excavation and there is provided within the liner a transverse member (4, 40) having therein an aperture (6) of limited dimensions through which the subsequent steps are performed, the aperture (6) forming a guiding and locating means by which the tools (T1, T2) used in those subsequent operations confine their actions to the required area (X) of the surface of the pipe (1).

2. A method according to claim 1 in which the transverse member (4) is a disc having a substantially central aperture (6).

3. A method according to claim 1 or claim 2 in which the axial position of the member (4, 40) within the liner (3) is capable of being selected over a range of positions.

4. A method according to any one of claims 1 to 3 in which the axial position of the member (4, 40) within the liner (3) is adjusted and set by means of at least one manipulating rod (5) extending upwards from the member (4, 40) to the neighbourhood of the upper end of the liner (3).

5. A method according to claim 4 in which the or each manipulating rod (5) is rotatable to actuate means (43) on the member for engaging in one of a number of vertically spaced detent means on the wall of the liner (3) .

6. A method according to claim 5 in which the detent means comprise a series of recesses (32-35) in the wall of the liner (3) engaged by a cam (43) actuated by the rod (5), and the rod is actuated to select the required recess to set the transverse member (4) at the desired level.

7. A method according to any one of claims 1 to 6 in which the aperture (Z) is formed in a plate (51) which is itself laterally adjustable in position in relation to the transverse member (40), and the plate is laterally adjusted if necessary to bring the aperture (Z) symmetrically over the exposed crown of the pipe (1).

8. A method according to any one of claims 1 to 7 in which the transverse member (4, 40) is guided against rotation in the liner (3) by the provision of at least one longitudinal rib (30, 31) in the liner (3) engaged by a notch (41, 42) in the transverse member.

9. Apparatus for putting into practice the method according to any one of claims 1 to 8 comprising a cylindrical liner (3) of diameter and length suitable for insertion in an excavation that exposes an underground pipe (1), a transverse member (4) capable of insertion in the liner and means (43, 32-35) for locating the member (4) in the liner (3) at any one of a range of longitudinal positions in the neighbourhood of one end of thereof, and manipulating means (5) for operating said locating means and operable from the neighbourhood of the other end of the liner, the transverse member (4) having therein an aperture (6, Z) acting to define and restrict the lateral limits of movement of a tool inserted through it.

10. Apparatus according to claim 9 including a plate, (51) the aperture (Z) being formed in the plate (51) and the plate being capable of being shifted laterally in relation to the transverse member (40) and located in selected positions (52, 53).